The capability of monitoring the optical performance of installed optical networks is desired for maintenance and diagnostic purposes. Optical signal-to-noise ratio (OSNR) is a key optical performance indicator, and monitoring the OSNR evolution of an optical transmission link can provide vital information about its performance. In modern optical networks, such as wavelength division multiplexing (WDM) networks, channels may be individually added and dropped at any location. If the OSNR is to be monitored within each channel band and the channel bandwidth is not much broader than the signal bandwidth (as in the case of dense WDM), conventional optical spectral measurement techniques for OSNR monitoring become unreliable because it is difficult to distinguish the signal spectrum from the noise spectrum in such a channel.
Several in-band OSNR monitoring techniques have been proposed to address the aforementioned problem. One technique is based on a polarization-nulling method, as described in J. H. Lee, et al., “OSNR monitoring technique using polarization-nulling method,” IEEE Photonics Technology Letters, Volume 13, Issue 1, 2001, pp. 88-90. The technique described therein assumes that the signal is fully polarized and that the noise is fully un-polarized. Unfortunately, this technique becomes unreliable in the presence of polarization-mode dispersion (PMD) and partially polarized noise, although its robustness against PMD can be improved with a special filtering technique, as explained in M. H. Cheung, et al., “PMD-insensitive OSNR monitoring based on polarization-nulling with off-center narrow-band filtering”, IEEE Photonics Technology Letters, Volume 16, Issue 11, 2004, pp. 2562-2564.
To address the aforementioned shortcomings and hence enhance the robustness and accuracy of OSNR monitoring, another method based on a phase modulator embedded fiber loop mirror has been proposed which is insensitive to PMD, chromatic dispersion (CD), and the degree of polarization (DOP) of the noise, as described in Y. C. Ku, et al., “A novel robust OSNR monitoring technique with 40-dB dynamic range using phase modulator embedded fiber loop mirror,” OFC'2006, paper OWN6. Disadvantageously, however, the OSNR monitoring method described therein is quite sophisticated and requires several expensive elements such as a phase modulator, a polarization scrambler, and a fiber loop mirror. Moreover, the measurement time needed to reliably measure the OSNR is long due to the need to align the polarization state of the incoming signal to a certain direction by the polarization scrambler.